The present invention provides an apparatus and method of measuring gas volume fraction in a process flow or fluid, such as slurries used in the paper and pulp industries and in other industries. Slurries commonly used in the paper and pulp industry are mostly water and typically contain between 1% and 10% pulp content by mass. Monitoring the gas volume fraction of a slurry can lead to improved quality and efficiency of the paper production process.
Processes run in the paper and pulp industry can often, either intentionally or unintentionally, entrain gas/air. Typically, this entrained air results in measurement errors in process monitoring equipment such as volumetric flow measurements and consistency meters.
Industry estimates indicate that entrained air levels of 2-4% are common. Since most process flow monitors are unable to distinguish between air and liquid, interpreting their output as liquid flow rates would result in a overestimate of the liquid by the volumetric flow rate of the air present at the measurement location. Similarly, for the void fraction of the air within the pipe can cause errors in consistency measurements.
Thus, providing a method and apparatus for measuring entrained air in paper and pulp slurries, for example, would provide several benefits. Firstly, it would provide a means to screen the output of process instrumentation. Secondly, in addition to screening the measurements, an accurate measurement of the entrained air would provide a means to correct the output of volumetric flow meters and consistency meters. Thirdly, monitoring variations in the amount of entrained air in a given process could be indicative of process anomalies, such a worn bushing or cavitating pumps and/or valves.
Multiphase process flow rate is a critical process control parameter for the paper and pulp industry. Knowing the amounts of liquid, solids and entrained gases flowing in process lines is key to optimizing the overall the papermaking process. Unfortunately, significant challenges remain in the achieving accurate, reliable, and economical monitoring of multiphase flow rates of paper and pulp slurries. Reliability challenges arise due the corrosive and erosive properties of the slurry. Accuracy challenges stem from the multiphase nature of the slurries. Economical challenges arise from the need to reduce total lifetime cost of flow measurement, considering installation and maintenance costs in addition to the initial cost of the equipment.
Currently, there is an unmet need for multiphase flow measurement in the processing industry, such as the paper and pulp industry. Real time flow measurement is typical restricted to monitoring the total volumetric flow rate in a process line without providing information on the composition of the process mixture. For example, electromagnetic flow meters are the most widely used flow meters in the paper and pulp industry, however they provide no indication of presence of entrained air, with its presence resulting in an over prediction of the volumetric flow of process fluid by the amount of air entrained. Consistency meter provide a measurement of the percentage of solids within the process, however this technology remains more of an art than a science. Furthermore, although entrained air is known to have a large, often deleterious, impact on the paper making process, instrumentation is currently not available to provide this measurement on a real time basis.
In one embodiment of the present invention, the apparatus and method improves the determination of consistency of paper and pulp slurries. Consistency refers to the mass fraction of pulp contained in water and pulp slurries used in the paper making process. Consistency measurements are critical in the optimization of the paper making process. Currently, many companies produce consistency meters employing various technology to serve the paper and pulp industry. Unfortunately, accurate and reliable measurement of consistency remains an elusive objective. Typically, interpreting the output of a consistency meter in terms of actual consistency is more of an art than a science.
Of the various types of consistency meters on the market, microwave based meters may represent the best the solution for many applications. One such microwave-based consistency meter is manufactured by Toshiba. Microwave consistency meters essentially measure speed or velocity the microwave signal propagates through the medium being measured. For example, the speed of the microwave signal through water is approximately 0.1 time the speed of light in a vacuum (c), through air is approximately 1.0 times the speed of light in a vacuum, and through fiber (or pulp) is approximately 0.6 times the speed of light in a vacuum.
The velocity of the microwave signal propagating through the paper pulp slurry is measure by the conductive effects of the slurry, in accordance with the following equation:V=c*sqrt(E)
Where V is the velocity of the microwave signal propagating through the slurry, c is the speed of light in a vacuum, and E is the relative conductivity of the material. Typical values of relative conductivity for material comprising a paper/pulp slurry, for example, are:                Water relative conductivity=80;        Air relative conductivity=1; and        Fiber relative conductivity=3.        
These meters typically work well in the absence of entrained air. With entrained air present, the air displaces water and looks like additional pulp fiber to the microwave meter. Thus, uncertainty in the amount of entrained air translates directly into uncertainty in consistency.